Magnesium molybdates have been prepared from an aqueous solution of magnesium nitrate and ammonium paramolybdate under various pH conditions and used for the selective oxidation of propane and isobutane with gaseous oxygen under atmospheric pressure in the temperature range of 360-520(degrees)C. The structure analyses by XRD and FT-IR showed the formation of three phases, alpha-MgMoO4, beta-MgMoO4 and Mg2Mo3O11, in the catalysts calcined below 550(degrees)C. The catalyst prepared at pH=5.7 showed the highest activity for the oxidative dehydrogenation of the alkanes as well as the strongest acidity. By XPS measurements, an excess amount of Mo compared to Mg was observed over the active catalysts. It is likely that the excess molybdenum species is present as molybdate and creates acidic sites over the catalyst surface.